1. Field of the Invention
This invention relates to electromechanical actuators in photographic cameras, and more particularly to such actuators that are easily adapted for control by digital computer.
2. Discussion Related to the Problem
The latest direction in photographic camera control is toward the use of a small digital computer, called a microprocessor, to control all camera functions. The microprocessor accepts digital imputs from a variety of transducers in the camera, such as scene light measuring apparatus, camera mechanism position indicators, automatic range finders, and the switch or button that is actuated by the photographer to initiate an exposure. The microprocessor responds to these imputs to produce control signals for various parts of the camera, such as lens focus, aperture size, shutter operation, mirror movement, and film advance. The control signals, as produced by the miroprocessor, are in a digital format; thus necessitating the step of digital-to-analog conversion when an analog output transducer is employed. Wherever possible, it is desirable to eliminate the step of digital-to-analog conversion by employing a transducer that can be driven directly by a digital signal, thereby simplifying the camera mechanism and reducing the overall cost. The search for simpler, more compact, and reliable electromechanical actuators, capable of being driven directly by digital signals, for moving elements such as aperture and shutter blades in photographic cameras, is being actively pursued.
U.S. Pat. No. 4,024,552 issued May 17, 1977 to Toshihiro Kondo, discloses a relatively simple electromechanically actuated optical blade comprising a rectangular planar conductive coil embedded in an opaque plastic blade. The blade is slidably mounted in grooves in the camera body, and one of the legs of the rectangular coil resides in the vicinity of a magnetic field that is directed perpendicular to the plane of the coil. When a current flows in the coil, a force, generally perpendicular to the one leg of the rectangular coil, is generated; causing the blade to slide in the grooves. Return springs are provided to return the blade to its starting position when the current ceases. Flexible extension leads supply the current to the coil from a source within the camera. In one disclosed embodiment, a pair of blades having triangular shaped cutouts cooperate to form progressively larger apertures as the blades move away from each other. The pair of blades are employed as a combination shutter and aperture; and the aperture size is determined by interposing adjustable stops in the paths of the blades.
Unfortunately, mounting a blade in a groove results in relatively high frictional forces if the blade should become even slightly canted in the groove due to any imbalance of force on the blade. For small coils and magnets, and using reasonable amounts of current, as required for compact camera applications, the electromagnetic force on the blade is marginally sufficient to reliably move the blade against the frictional forces retarding its movement. In addition, the flexible extension leads provide further resistance to movement of the blade. The use of adjustable stops to determine aperture size when the optical blade is used as a combination shutter/aperture complicates a mechanism whose primary virtue is simplicity.
The present invention is directed to an improved method of controlling a planar electromechanical actuator and an improved actuator that solves the problems noted above.